Drawbacks of the State of the Art
A. Rigid Cuff Electrodes
Extraneural, cuff-like electrodes, so called "cuff electrodes" which are placed circularly around the nerve, have been a part of successful biomedical electrodes on the market since the end of the 1970s. There are a number of US patents for rigid construction, so-called "split cylinder", cuff electrodes: U.S. Pat. No. 3,774,618; U.S. Pat. No. 3,738,383; and U.S. Pat. No. 3,654,933. Some implanted electrodes have been in safe use for 15 years. Their disadvantages were, and still are, their limited selectivity due to the small number of electrodes (circular maximum 4). The arrangement of three electrodes in the longitudinal direction of the cuff, and the use of transverse control currents, permits improving the selectivity and stimulation of deeper lying parts of nerves. However, their relatively rigid form may damage the nerves in the event of post-operative edematous swelling due to the pressure of the electrode on the nerves. If the diameter of the cuff is selected to be larger, then conduction and stimulation conditions are considerably impaired due to the ingrowing of the tissue.
B. Flexible Cuff Electrodes
The introduction of the so-called spiral cuff electrodes constituted an approach to reduce postoperative increases in pressure. In this mode of construction, a prestretched silicon foil is connected to an unstretched foil using a medical silicon adhesive. The electrodes are composed of platinum wires which are placed between the foils. The active electrode surfaces are created by opening windows (using, for example, a scalpel or punching iron) in the prestretched foil.
Due to the tensile stress of the prestretched foil, the electrode curls, thus becoming a spiral. When the nerves swell, the spiral-shaped cuff electrode can stretch and thereby partially compensate for the increase in pressure.
C. Helix Electrodes
The term helix electrode refers to electrodes that are wound screw-like around the nerve. They are employed in monopolar and bipolar form with a maximum of two electrodes. Due to the open construction of the electrode, the danger of the pressure damaging the nerves by swelling is minimal. However, field definition on the interior of the electrode when stimulated (less energy needs) is not possible. The helix electrodes disadvantageously have no way of selectively stimulating specific parts of the nerve cross section.
The object of the present invention is to provided an improved cuff electrode. In particular, the object is to reduce the pressure placed on the nerves by the electrode. A bidirectional coupling to the nerves with high spatial and temporal resolution has the purpose of permitting diversion of the nerve signals and/or electric stimulation of nerve fibers.